1. Field of the Invention
The present invention relates to the refining of aluminum and, more specifically to a system for degassing molten aluminum without significantly reducing magnesium levels, if present.
2. Prior Art
Molten aluminum typically is contaminated with hydrogen and unwanted elements such as sodium, calcium, and lithium. If magnesium is present in the melt to an excessive extent, the excessive magnesium likewise can be regarded as a contaminant; however in working with many aluminum melts, the level of magnesium that is present is not excessive, and it is desirable to effect removal of hydrogen and other unwanted elements without significantly altering the level of magnesium that is present in the melt.
A problem with prior processes for removing hydrogen and the other contaminants mentioned above is that the magnesium content of the melt often is altered undesirably as the processes are preformed.
Prior proposals to remove the aforementioned contaminants have included the use of gases bubbled up through the melt to chemically react with the impurities or to physically remove the impurities. Suitable gases include inert gases, such as argon and nitrogen, reactive gases such as chlorine (from chlorine-containing salts, chlorine gas generating tablets, typically hexachloroethane, chlorine gas and halocarbon gases), and fluorine, as well as mixtures of inert gases (nitrogen and argon) or mixtures of reactive gases (chlorine and fluorine).
The patent literature contains proposals for removing specific contaminants. Small quantities of alkali metal and alkaline earth metal impurities are removed from an aluminum melt by introducing a source of sulfur, such as elemental sulfur, according to a process that is described in U.S. Pat. No. 4,354,869. An impurity-containing slag is formed and is removed form the melt. Lithium is removed from molten aluminum alloys by introducing sulfur hexafluoride as a gas or with a carrier gas such as nitrogen or argon according to Austrian Pat. No. 354,114. The fluorine in the SF.sub.6 reacts with the lithium to form a LiF precipitate, while the sulfur reacts with the lithium to form Li.sub.2 S. The lithium content of the alloy is reduced to the parts per million level. Gaseous and solid impurities such as occluded hydrogen and metal oxides are removed from molten aluminum using a fully fluorinated or chlorinated lower hydrocarbon mixed with a relatively inactive or inert gas, as is described in U.S. Pat. No. 3,854,934. A liquified salt cover, lower in density than the molten aluminum, floats on the metal surface to inhibit discharge of potentially harmful gases.
Three physical mechanisms are active in the removal of these elements. When a gas of the type described is introduced into the melt, hydrogen that is present in the melt diffuses into the rising gas bubbles, and slag particles adhere to the surface of the gas bubbles--which causes the slag particles to be flushed up to the surface of the bath. In addition, the elements to be removed may react with one or more of the reactive components of the gas introduced into the melt.
Hydrogen is removed from the melt by diffusion into the rising gas bubbles. This occurs as a result of the difference in partial pressure between the melt and the gas, the rate of diffusion is determined by the partial pressure difference between the gas and the melt, as well as by the surface area of contact between the gas and the melt. The contact time between the gas and the melt is also an important consideration.
A gas which bubbles through a melt must have as low a hydrogen or moisture content as possible. If the partial pressure of the hydrogen in the gas is assumed to be about zero, the difference in hydrogen partial pressure will be substantial at the beginning of the flushing. The large difference in pressure forces hydrogen from the melt into the gas bubble. This means that hydrogen easily diffuses into the gas bubbles and that large quantities of hydrogen are removed.
The partial pressure of hydrogen in the melt decreases with increased flushing time, and as the difference in partial pressure decreases, the removal of the hydrogen diminishes. This means the flow of the gas used for degassing should be increased at the end of the process to facilitate the removal of the final amounts of hydrogen. Thus while the hydrogen content in a melt containing a large amount of hydrogen can be quickly decreased, further reduction of the hydrogen content will require a long flushing time and relatively large quantities of gas.
The surface area of the gas bubbles into which the hydrogen diffuses determines the area of contact surface between the melt and the inert gas; thus a larger contact surface area will have a positive influence on the removal of the hydrogen. A larger contact surface area is obtained by increasing the amount of gas, or by decreasing the size of the bubbles. The amount of gas supplied to the melt may be increased by utilizing a longer flushing time, or by utilizing a greater volume of gas per time unit. A disadvantage of utilizing an increased flow of gas is that the gas flow through the melt causes a temperature drop, and the temperature drop that results with an increased flow likely will be greater than desired. Further, the increased flow of gas likely will create such violent movement in the bath that particles from the surface of the bath may be pulled down into the melt.
For reasons of economy, it is desirable to use as little gas as possible. Thus, it is preferable to reduce the size of the bubbles so that the surface area to volume relationship is improved.
Apart from the difference in the hydrogen partial pressure and the size of the bubbles, the diffusion of hydrogen into the gas bubble is also time dependent. This means that decreasing bubble size serves a double function. In addition to improving the surface area to volume relationship, smaller bubbles tend to rise more slowly in the melt, thus providing a longer contact time with the melt.
The average contact time that bubbles spend moving through a melt is also dependent on the geometry of the furnace. With a given melt volume, increased bath depth will have a positive influence on the flushing effect since the gas bubbles will then be in contact with the melt for a longer period of time.
The direction in which bubbles are injected into a melt also will have a marginal effect on contact time. If gas is injected in a downward direction, the bubbles will be forced down into the melt before they begin to rise to the surface, whereby contact time will be increased.
Oxides and nonmetallic inclusions adhere to the bubble surface and are flushed out of the melt via flotation Particles in the size range of 1 micrometer to 1 millimeter that are suspended in the melt readily attach themselves to the surfaces of the upward rising bubbles and are given sufficient upward movement to be flushed to the surface of the bath. Flushing is influenced by the size of the bubbles, the route the bubbles take, and the surface tension of the bubble.
The degassing agent used can cause a wetting effect on the bubble surface which will increase the ability of the gas to remove oxides and particles. Bubble size and path are an expression of the likelihood that a particle will be encountered by a bubble; surface tension determines to what extent a particle will adhere to the bubble which it encounters.
The unwanted elements in aluminum melts normally concerned are sodium, calcium and lithium. Another element that is present in aluminum melts, but which often is desirably left in the melt, is magnesium. While the removal of these elements has been widely discussed, the mechanisms of their removal are not fully understood.
There are three basic methods in commercial use for removing impurities from aluminum melts. One method uses argon. A second uses nitrogen. A third uses chlorine.
Using the inert gases argon and nitrogen is effective to remove hydrogen, at least to a certain level, but is relatively ineffective in removing active species impurities such as sodium, calcium, lithium and excessive magnesium. A more detailed discussion of the relative advantages and disadvantages of the use of the inert gases, argon and nitrogen, and the reactive gas, chlorine, will conclude this discussion of the practices of the prior art.
Argon and nitrogen work fairly well for degassing, but they can only remove up to a certain absolute level of remaining hydrogen. A further problem with industrial grades of argon or nitrogen is that these gases can contain moisture and oxygen which can form hydrogen and aluminum oxides; an ultra high purity gas avoids this problem. Neither nitrogen or argon have much of an effect on oxides or particles present in the melt.
Argon is an inert gas, i.e., it does not react with the melt, thus hydrogen is removed by diffusion into the argon bubbles. Particles are removed by the purging mechanism. Argon has no effect on the elements apart from a possible mechanical agitation effect.
In principle, nitrogen acts in the same way as argon. It is, however, generally accepted that, under similar circumstances, the removal of hydrogen takes place somewhat quicker when using argon that nitrogen. Also, the absolute value of the hydrogen remaining in the melt will be slightly lower when using argon than when using nitrogen.
Nitrogen gives a wetter slag than argon and can be a problem in alloys containing more than 1% magnesium. This is because nitrogen is not completely inert, but reacts with the melt to form nitrides, particularly Mg.sub.3 N.sub.2.
Chlorine added to the melt quickly reacts to AlCl.sub.3 which is gaseous at temperatures above about 374 degrees Farenheit, so that, in reality, the melt is flushed by upward rising AlCl.sub.3 bubbles. Chlorine is extremely effective for removing hydrogen, which is removed by diffusion, because the hydrogen partial pressure in AlCl.sub.3 is virtually zero. However, chlorine gas is used in a stoichiometric excess and exits the melt as pure chlorine which presents safety and environmental concerns. In addition, chlorine reacts rather slowly with aluminum as compared with other metals.
AlCl.sub.3 gas bubbling through the melt is reactive and continues to react with the melt. Salt particles formed collect like slag on the surface of the melt, but some remain suspended in the melt. It has been shown that these particles can be the cause of the creation of agglomerates (an aluminum droplet encrusted with a salt film to which small particles of magnesium oxide and aluminum nitride are adhered) with diameters of about 20 to 200 micrometers.
Chlorine reacts with the elements sodium, calcium, magnesium, lithium, etc., and with aluminum; but where magnesium removal is not desired, this presents a problem. Chlorine removes substantial quantities of magnesium and in consequence, the magnesium must often be replaced. Fluorine acts in a manner similar to chlorine.
Chlorine and fluorine are available in several forms, the most common of which are chlorine gas, salts, and halocarbons. Dry chlorine gas gives the same effects as described for chlorine. Hexachloroethane salt produces the same reaction as described for chlorine and/or fluorine, but the salt is very often hygroscopic. This means that salt can introduce moisture into the melt depending on how the salt has been stored, the relative humidity at the time of storage and/or use, and the type of salt.
The effect of a halogen releasing salt on the hydrogen content of the melt depends upon the reactions which create gas bubbles. The creation of bubbles is uncontrolled after the addition of the salt--bubbling begins violently and then tapers off as the salt is consumed, which is inconsistent with the need for greater quantities of gas at the end of the degassing process in order to remove the last ppm of hydrogen. Halocarbons are normally introduced into the melt in the form of a gas which also have the same reactions as described above for chlorine and fluorine. In addition to the usual reactions as with chlorine and fluorine, reactions with the carbon component of the halocarbon gas also take place.
Sulfur hexafluoride (SF.sub.6) can also be added as a gas to the melt. As with chlorine and fluorine, sulfur hexafluoride can successfully cope with diffusion and flushing; however, reaction is minimal. Sulfur hexafluoride uses the flushing action more efficiently to rival the quality obtained by the reaction of chlorine and fluorine.
Safety of these gases and gas-producing materials is a concern. Although nitrogen and argon are nontoxic, chlorine (and fluorine) gas is corrosive and poisonous and strict limits are placed on worker exposure to chlorine as well as transport and storage of the gas. Halocarbon gases are virtually nontoxic and noncorrosive; however, many can replace oxygen in the air presenting a suffocation potential for those near the gas. Certain halocarbons decompose at degassing temperatures.
Sulfur hexafluoride is a similar to halocarbon 12 in that it is a nontoxic, noncorrosive gas. The time threshold limit value-TWA ("TWA" stands for "time weighted average") for sulfur hexafluoride is 1000 ppm. As with halocarbon 12, it can replace the oxygen in the air so there is a danger of suffocation. Sulfur hexafluoride does not decompose as does halocarbon 12; it is more stable at high temperatures and is often used to blanket high temperature operations. Any breakdown into sulfur and fluorine is immediately consumed by aluminum and is flushed to the surface.